The claimed invention relates generally to the field of disc drive data storage devices and more particularly, but not by way of limitation, to a method and apparatus for accelerating a disc drive spindle motor from rest to a final operational velocity.
A disc drive is a data storage device used to store digital data. A typical disc drive includes a number of rotatable magnetic recording discs which are axially aligned and mounted to a spindle motor for rotation at a high constant velocity. A corresponding array of read/write heads access tracks defined on the respective disc surfaces to write data to and to read data from the discs.
Disc drive spindle motors are typically provided with a three-phase, direct current (dc) brushless motor configuration. The phase windings are arranged about a stationary stator on a number of radially distributed poles. A rotatable spindle motor hub is provided with a number of circumferentially extending permanent magnets in close proximity to the poles. Application of current to the windings induces electromagnetic fields which interact with the magnetic fields of the magnets to apply torque to the spindle motor hub and induce rotation of the discs.
Due to the prevalence of numerous consumer devices that employ electrical motors, it might seem at first glance that accelerating a disc drive spindle motor from rest to a final operational velocity would be relatively straightforward; simply turn on the motor and let it accelerate to the final desired speed. As those skilled in the art will appreciate, however, just the opposite has proven to be the case. Accelerating a spindle motor from rest can be fraught with difficulty and involves a number of important considerations that must be adequately taken into account.
First, it is important to accurately determine the rotational state of a disc drive spindle motor prior to application of drive signals to the motor. Application of drive signals to a spindle motor while the motor is in an unknown state could lead to the inadvertent rotation of the motor in the wrong direction. Rotating the spindle motor in the wrong direction, even for a very short time, can lead to premature failure of a disc drive; heads and disc surfaces can be damaged, and lubricating fluid used in hydrodynamic spindle motor bearings can be pumped out of the bearings.
Early disc drive spindle motor designs used Hall effect or similar external sensors to provide an independent indication of motor positional orientation. However, present designs avoid such external sensors and instead use electronic commutation and back electromagnetic force (bemf) detection circuitry to provide closed-loop spindle motor control, such as discussed in U.S. Pat. No. 5,631,999 issued to Dinsmore. Such approach generally entails applying a predetermined sequence of commutation steps to the phase windings of the spindle motor over each electrical revolution (period) of the motor. A commutation step involves supplying the motor with current to one phase, sinking current from another phase, and holding a third phase at a high impedance in an unenergized state.
Detection circuitry measures the bemf generated on the unenergized phase, compares this voltage to the voltage at a center tap of the windings, and outputs a signal at a zero crossing of the voltages; that is, when the bemf voltage changes polarity with respect to the voltage at the center tap. The point at which the zero crossing occurs is then used as a reference for the timing of the next commutation pulse, as well as a reference to indicate the position and relative speed of the motor.
Above an intermediate operational speed, the control circuitry will generally be able to reliably detect the bemf from rotation of the spindle motor, and will further be able to use the detected bemf to accelerate the motor to a final operational velocity. Below this intermediate speed, however, closed-loop motor speed control using detected bemf generally cannot be used since the spindle motor will not generate sufficient bemf at such lower speeds.
Thus, a related difficulty encountered in accelerating a disc drive spindle motor from rest is getting the motor to properly and safely rotate up to the intermediate velocity so that the closed-loop motor control circuitry can take over and accelerate the motor up to the operational velocity.
Several approaches have been proposed in the prior art to accelerate a disc drive spindle motor from rest to an intermediate velocity, such as exemplified by U.S. Pat. No. 5,117,165 issued to Cassat et al. This reference generally discloses determining the electrical rotational position of a spindle motor to determine the commutation state of the motor; that is, to determine the appropriate commutation pulses that should be applied to accelerate the motor based on the then-existing motor position. Drive pulses of fixed duration are applied to the motor to induce torque and initiate rotation of the motor, and the electrical rotational position of the motor is measured between application of each successively applied, fixed duration pulse.
Once the motor rotates sufficiently to induce a change in commutation state, the next set of drive pulses are applied, and position measurements are taken between the application of each set of the drive pulses as before. As the motor achieves a higher rotational velocity due to the successive xe2x80x9cnudgingxe2x80x9d provided by the drive pulses, the time between successive commutation states becomes shorter, decreasing the number of drive pulses applied during each commutation state.
Eventually, an upper limit on the achievable rotational velocity will be encountered using this approach. This upper limit is generally reached as the combined time for the drive pulses and position measurement approaches one half the commutation time. As the motor velocity approaches this upper limit, an uneven, cogging action will typically be induced in the motor because the drive pulses are not synchronized with the motor rotation; that is, the drive pulses are not applied when the windings and magnets are optimally aligned for each new commutation state. Such operation does not generally harm the motor, but does result in less than efficient operation and limits the torque that can be applied to the motor. This cogging action ultimately acts as a velocity governor and undesirably induces variation in the rotational velocity of the motor.
The final velocity achieved by this approach must be high enough to enable a hand off to the motor control circuitry; that is, the final velocity must be high enough to enable the spindle motor to generate bemf that can be detected and used by the bemf detection circuitry. However, the particular velocity at which bemf is reliably generated is a function of the motor construction, and recent generation high performance spindle motor designs with higher operational velocities and fewer numbers of poles have been found to require a higher intermediate velocity before sufficient bemf is generated to allow frequency lock by the motor control circuitry.
Moreover, the motor speed variation increases as the motor velocity reaches the upper limit, and such variation makes it more difficult for the motor control circuitry to obtain frequency lock on the spindle motor. Thus, as disc drive manufacturers implement spindle motor designs with ever higher levels of performance, it is becoming increasingly difficult for prior art motor start up routines to accelerate the spindle motors to a sufficient velocity to enable the motor control circuitry to take over and implement closed-loop acceleration up to the operational velocity.
Accordingly, there is a need for improvements in the art whereby a high performance spindle motor can be reliably accelerated from rest to an operational velocity. It is to such improvements that the present invention is directed.
In accordance with preferred embodiments, a disc drive includes a spindle motor, back electromagnetic force (bemf) detection circuitry which detects bemf from rotation of the spindle motor above an intermediate velocity, commutation circuitry which electrically commutates the spindle motor in relation to the detected bemf over a range of commutation states, and control circuitry which directs the acceleration of the spindle motor from rest to a final operational velocity.
During a low gear mode, the spindle motor is initially accelerated from rest to a first velocity by applying short, fixed duration drive pulses to the spindle motor. Each drive pulse is preferably followed by two quick position measurements. The drive pulses and measurements continue until a commutation transition is detected, after which a new set of drive pulses appropriate for the new commutation state (and position measurements) are applied.
Once the first velocity is reached, a high gear mode is employed wherein the spindle motor is accelerated from the first velocity to an intermediate velocity greater than the first velocity. Variable duration drive pulses are applied to the spindle motor and successive spindle motor commutation state transitions are detected. The variable duration of each successive drive pulse is established in relation to a most recent commutation period comprising the elapsed time between the two most recently detected state transitions. Only one variable duration drive pulse is preferably applied during each commutation state, after which position measurements are repeatedly made while the spindle motor coasts to the next state transition.
Thereafter, the spindle motor is accelerated from the intermediate velocity to the final operational velocity using the commutation circuitry and bemf detection circuitry. Zero crossings are detected in relation to bemf from the spindle motor and the zero crossings are used to time the application of subsequent commutation pulses to the motor.
Preferably, operation during low gear mode includes steps of identifying the initial commutation state of the spindle motor while the spindle motor is at rest, and repetitively applying a fixed duration drive pulse and measuring electrical rotational position of the spindle motor until a transition to the next commutation state is detected. The drive pulses and measurements are repeated until the first velocity is reached.
Operation during high gear mode preferably includes steps of measuring the duration of the most recent commutation period, calculating a drive pulse duration in relation to the duration of the most recent commutation period and a scale factor so that the drive pulse duration is less than the duration of the most recent commutation period, applying a drive pulse with the calculated drive pulse duration to the spindle motor, and repetitively measuring electrical rotational position of the spindle motor until a transition to the next commutation state is detected. The foregoing steps are repeated until the intermediate velocity is reached. The scale factor can be a constant, or can vary in relation to variations in the rotational velocity of the spindle motor. Once the intermediate velocity is reached, the spindle motor is accelerated to the operational velocity using back electromotive force (bemf) detection.
By accelerating the spindle motor in this manner, smooth and continuous transitions in spindle motor velocity are obtained, and cogging and reverse rotation of the spindle motor are avoided.
These and various other features and advantages which characterize preferred embodiments of the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.